In cultured human epidermal cells exposure to the vesicant sulfur mustard (HD) causes a decrease of the NAD+ content, which depends on the dose and the time period between exposure to HD and NAD+ measurement. Presumably, this NAD+ loss is due to activation of the enzyme NAD:protein ADP-ribosyltransferase (ADPRT) and may lead to glycolysis inhibition, disturbance of energy metabolism, and eventually cell death. Since prevention of this NAD+ depletion could lead to cell survival, HD-exposed cultures have been incubated with nicotinamide, a precursor of NAD+ and an inhibitor of ADPRT. Although a reduction in NAD+ levels of the cultures can be prevented, the uptake of glucose, which was taken as a measure for cellular viability, appears to be inhibited in cultures in which the NAD+ levels are at the 100% level at 4 hr after exposure. Therefore, prophylactic or therapeutic measures that are focused on maintenance of NAD+ levels in order to preserve energy supplies do not protect human epidermal cells in culture that have been exposed to HD. These experiments indicate that mechanisms other than NAD+ depletion may play an important role in HD-induced cell injury in human skin.